doc: fix broken internal documentation links

[zhanzhao-h]

Description

Fix broken internal Markdown links in the documentation.

Notes

• doc/README.md: point the Assumeutxo design link at doc/design/assumeutxo.md.
• README.md: point secp256k1 usage example links at src/secp256k1/examples.

Validation

• Checked the touched Markdown links resolve to files/directories in the repository.
• Ran git diff --check on the local patch.

Related to #32.

[zhanzhao-h]

Follow-up: added CONTRIBUTING.md to this PR as well. It fixes the remaining broken secp256k1 internal links for src/secp256k1/include/secp256k1.h and src/secp256k1/src/tests_exhaustive.c.

[MyTH-zyxeon]

Quick review assist for the #81 bounty/improvement queue.

I checked the touched Markdown links against the current default branch paths:

• doc/design/assumeutxo.md
• src/secp256k1/examples/ecdsa.c
• src/secp256k1/examples/schnorr.c
• src/secp256k1/examples/ecdh.c
• src/secp256k1/include/secp256k1.h
• src/secp256k1/src/tests_exhaustive.c

The diff is docs-only (CONTRIBUTING.md, README.md, doc/README.md) and the PR currently reports a clean merge state. Suggested maintainer acceptance check: render/open the changed Markdown links in GitHub and confirm no Bitgesell-specific docs were redirected to upstream-only paths by mistake.

No bounty claim from me here; this is just triage help to keep the open #81-related PR queue moving.

[zhanzhao-h]

The CI run failed without creating any jobs, and the logs are unavailable. I checked the workflow run and it reports total_count: 0 jobs; downloading logs returns 404.

Could a maintainer please re-run the workflow? I tried to re-run it from my side, but GitHub requires admin rights on the upstream repository for this run.